This invention relates to a method and apparatus for driving CCD image sensor in facsimile type equipment in which the signal sensed by the CCD image sensor is coded by using a redundancy suppressed coding technique.
In facsimile equipment using the above mentioned coding technique because the amount of coded information varies for each main scan, intermittent and variable speed subscanning systems have been adopted. In these subscanning systems, the subscan stops when the information stored exceeds a certain amount. The subscan then resumes when the information stored is less than a certain amount. However, in these subscanning systems, high speed operation is difficult because of the time it takes to completely stop subscanning. In addition, during the stopping or starting of the subscan, disorders sometimes occur in the reproduced picture due to vibration of the mechanical system.
Other variable speed subscanning systems have been adopted to resolve the above mentioned problems. In such systems, a predetermined number N of subscan speeds are available for selection. By selecting the proper combination of subscan speeds from the predetermined number N of subscan speeds, each main scan can be made to correspond to the amount of coded information.
Although the disadvantages of stopping and starting the subscan are avoided in the latter systems, other disadvantages occur as explained more fully below.
The exposure time period of a CCD image sensor is fixed at T seconds as shown in FIG. 1. When the first shift pulse P.sub.1 (in conventional CCD image sensors, shift pulses supplied to the CCD image sensor cause information stored in the image sensor to be shifted out so that new information can be stored in response to the next exposure) is supplied to the CCD image sensor, exposure a.sub.1 for particular main scan begins. When shift pulse P.sub.2 occurs, exposure a.sub.1 is finished and exposure a.sub.2 for the next main scan begins. Upon the occurrence of shift pulses P.sub.3, P.sub.4, P.sub.5, . . . , similar exposures a.sub.3, a.sub.4, a.sub.5, . . . , take place. Information stored in the CCD image sensor during exposure a.sub.1 is outputted in response to shift pulse P.sub.2. The transfer output time of the information stored during exposure a.sub.1 is represented by signal b.sub.1. Similarly, information stored during exposure a.sub.2 is outputted during the time represented by signal b.sub.2 in response to shift pulse P.sub.3. At the times of shift pulses P.sub.4, P.sub.5, P.sub.6, . . . , information stored during exposures a.sub.4, a.sub.5, a.sub.6, . . . , is outputted during transfer output times b.sub.4, b.sub.5, b.sub.6, . . .
As mentioned previously, in the above conventional system, the exposure time period is fixed at T seconds. As a result, the outputting of stored information signals can only occur in the interval of an integer multiple of T. This means that, when the information stored in the CCD image sensor cannot be outputted in one time period T, thereby necessitating further delay in the subscan time period, the subscan time period must be an integer times T, such as one T, 2T, 3T, etc., which is not a continuous value. When applied to a CCD image sensor in facsimile equipment, time is lost in outputting stored information from the CCD image sensor because of the fixed time period T in the subscan. As a result of this lack of control over the outputting of information during subscanning, the overall speed of the facsimile equipment is lessened.